Is it possible to use write.table() and ddply, together?

Question

Let\'s say I have a data.frame like:

a <- c(1:10,1:10,1:10,1:10,1:10,1:10,1:10,1:10,1:10,1:10)
df <- data.frame(a,rnorm(100))

And I

Answer 1

It's possible to do it with ddply, but that is not what the function was designed for. From the plyr documentation:

All plyr functions use the same split-apply-combine strategy...

You want to split the data.frame and apply a function but you don't want to return anything, so ddply will throw an error if you don't return something that can be combined into a data.frame.

Answer 2

Continuing from Joshua's answer, the plyr function to use is d_ply which does not expect to return anything. You can do something like this:

d_ply(df, .(a),
      function(sdf) write.csv(sdf,
                              file=paste(sdf$a[[1]],".csv",sep="")))

The file argument to write.csv is constructed such that each subset gets a different filename.